1. Field of the Invention
This invention relates to video display terminals and color display monitors used therein. More particularly, this invention relates to a novel conversion circuit for enabling the use of an eight color, color display monitor to display twenty-seven or more different colors.
2. Description of the Prior Art
Video display terminals (VDT) are commercially available with color display monitors which will display twenty-seven distinct colors. The twenty-seven different colors may be employed as either background or foreground colors. Presently, video display terminals (VDT's) are made by a large number of U.S. manufacturers. The color display monitors that are employed in these terminals are only made by a few manufacturers. Heretofore, it has been common practice for the manufacturers of color display monitors to specify the recommended red, green and blue (RGB) input voltage levels which will assure acceptable operation of the color display monitor.
Heretofore, manufacturers of eight-color display monitors (CDM's) specified two low voltage logic levels as input signals for the RGB video input lines. The eight-color CDM's are presently manufactured by several competitive suppliers and have been standardized in this industry. Presently, there are only a few manufacturers of twenty-seven color CDM's, such as Mitsubishi, and the low voltage logic levels of the inputs to the twenty-seven color CDM's have not been standardized. Further, the twenty-seven color CDM's presently available are expensive in comparison to the standard eight-color CDM's. It is believed that a large portion of the extra cost is due to the complexity of the amplifiers and processing circuits at the input of the CDM's of the color cathode ray tube. For example, six lines having two binary logic levels are capable of defining sixty-four distinct conditions. To produce a twenty-seven color CDM, only twenty-seven of the sixty-four possible conditions need to be employed.
Presently, the six lines from the cathode ray tube (CRT) controller are applied to twenty-seven color CDM's which employ amplifying and processing circuits which produce three voltage levels on each of the three RGB video input lines to the color cathode ray tube. Production of three voltage levels on a single line requires rather complex circuitry and employs an analog mode of operation. For example, there is provided in the prior art a high level pulse, a half level pulse and a no level pulse which defines the intensity of the electron beam of the RGB video input lines. The different colors of the prior art twenty-seven color CDM's are produced on the color cathode ray tube screen by applying different intensity signals to the RGB video input lines for the same time duration. It has been observed that the elimination of the requirement for three or more voltage levels to define the intensity of the RGB beams would be highly desirable.